In recent years, urea greases have come into use in a wide range of fields since they have higher dropping points than general purpose lithium soap greases wherein lithium soaps are used as thickeners, have excellent thermal stability and excellent abrasion resistance and lubricating properties.
In the vehicle industry, there is a progressive increase in cases where the superior performance of urea greases is utilised because of the higher values required for the heat resistance, abrasion resistance and frictional properties of various vehicle parts, including CVJ (constant velocity joints). However, because of the striking advances in automobile technology and the year by year increase in the values required for each individual vehicle component, simply to maintain the status quo will not do.
In particular, the technical innovation in automobile electric power steering devices is remarkable, such that these devices, which were initially only used in some solar cars and light automobiles, are now very widely installed in small to medium-sized passenger cars. This is a vigorously growing sector wherein the number of such devices installed is almost doubling every year.
At present, the mainstream automobile power steering devices are of the hydraulic type. However, with such hydraulic power steering devices, environmental problems due to the use of hydraulic oil (power steering fluid) must be taken into account. There is an associated loss in engine power when such hydraulic power steering devices are installed because the hydraulic pressure pump required to create the oil pressure is driven by power from the engine, and is continuously driven (even when the steering wheel is not operated). Consequently, this is a factor causing a deterioration in fuel consumption.
In contrast, in electric power steering devices an electric motor is used as the power assist power source. By means of a control unit, it is possible to drive the electric motor only at times when the power assist is necessary. Moreover, since the electric motor drive uses electricity generated when the car is running, the engine power loss is very small. Accordingly, there is a substantial fuel economy effect, and energy consumption is greatly decreased compared to the hydraulic power steering devices.
However, since the power output generated by the current electric power steering devices is still low compared to that from hydraulic power steering devices, it is important not only to increase the electric motor power but also to decrease the load on the motor to the maximum extent by reducing friction among individual component parts as much as possible.
Furthermore, particularly in cold regions, the low temperature starting properties of the electric power steering devices are a major factor. In hydraulic power steering devices, when the engine warms up, the hydraulic pump directly connected to the engine has the effect of warming up each part of the steering device with the hydraulic oil acting as a heat transfer medium. Hence, normal low temperature characteristics were satisfactory for the lubricants used in such devices. However, in the case of electric power steering devices, there is no direct heat source from the engine, and the steering device cannot readily be warmed up.
Consequently, it is essential that the grease used for the components in electric power steering devices should display stable low friction torque properties.
Furthermore, since vehicles are used throughout the world, electric power steering devices which are designed and manufactured to take account of extremely cold conditions of about −40° C., may also face regular use at temperatures of 100° C. or more (arising from heat radiation in engine space and heat radiated from road surface).
Consequently, there is a demand for greases having a long life corresponding to the vehicle lifetime, which greases provide stable low torque properties over a wide temperature range from low to high temperatures, and with which oil film breakdown due to decreased viscosity at high temperature does not occur.
Electric power steering devices are broadly classified into three types: (i) column assist electric power steering devices, (ii) pinion assist electric power steering devices, and (iii) rack assist electric power steering devices.
The decelerator in column assist electric power steering devices and pinion assist electric power steering devices is usually made up of a metal worm gear and a resin worm wheel, and the power assist is effected by transmission of power from the electric motor to the power assist via these gears. At the gear contact areas, sliding friction between resin and metal occurs, and a lubricant is applied to these places.
Japanese Laid-Open Specifications 2001-64665, 2002-308125, 2002-363589, 2002-363590, 2002-371290 and 2003-3185 are literature references relating to lubricants used for prior art column and pinion electric power steering devices.
Japanese Laid-Open Specification 2001-64665 describes automobile steering grease compositions which contain (a) a thickener, (b) a base oil of pour point −40° C. or below, (c) an organic molybdenum compound, (d) melamine isocyanurate, (e) polytetrafluoroethylene and (f) molybdenum disulphide. Said reference discloses that the automobile steering grease compositions exhibit suitable lubricating properties at the engagement areas of gears such as in particular the rack and pinion parts or pinion assist electric power steering hypoid gears. However, those grease compositions are entirely different to the grease composition of the present invention.
Japanese Laid-Open Specification 2002-308125 describes an electric power steering device using as lubricant a grease which has improved lubrication durability at high temperatures, while maintaining a low starting rotational torque at low temperatures. Said reference states that in said grease the base oil is a synthetic hydrocarbon oil, the thickener is selected from a lithium-based complex soap or a urea compound and the lubrication enhancer is selected from a solid lubricant or an oil. Said reference discloses that the electric power steering device has an electric motor to generate the steering assist force and a deceleration device which reduces the rotation speed by means of a gear mechanism connected to the rotating shaft of the motor, at least one deceleration gear of that gear mechanism is made of a synthetic resin, and that synthetic resin gear is lubricated by the grease. However, although a urea thickener is mentioned in the claims concerning said grease composition, there is no disclosure whatever of its specific composition and effects.
Japanese Laid-Open Specification 2002-363589 describes a lubricating grease composition containing a base oil and a thickener, wherein a fluorinated resin powder is blended into the base oil. Said reference indicates that the lubricating grease composition may be used in the decelerators of electric power steering devices and the like.
Japanese Laid-Open Specification 2002-363590 describes a lubricating grease composition containing a base oil and a thickener, wherein Li stearate and Li hydroxystearate are used together, and indicates that said lubricating grease composition may be used in the decelerators of electric power steering devices and the like.
Concerning the decelerators of the electric power steering devices described Japanese Laid-Open Specifications 2002-363589 and 2002-363590 and their specific lubricated regions, it is disclosed for example that these are decelerators using worm wheels made of a synthetic resin such as a polyamide resin, and that the role of the lubricating grease compositions contributing to the reduction of friction in the lubricated regions of the sliding parts (friction surfaces) of both the synthetic worm wheel and the metal worm shaft is important. However, the grease compositions of said references are completely different from the grease composition of the present invention.
Japanese Laid-Open Specification 2002-371290 describes a resin lubricating grease composition containing a thickener and a base oil, and wherein montan wax has been incorporated. Said reference discloses that the decelerators of the electric power steering devices and the specific lubricated parts thereof are the resin (polyamide) worm wheel gear and steel worm gear decelerator mechanism parts. However, although a urea thickener is mentioned in the claims concerning said grease composition, it is of note that montan wax is described as an essential component and the grease composition is a resin lubricating composition. Accordingly, the grease composition of said reference differs completely from the grease composition of the present invention.
Japanese Laid-Open Specification 2003-3185 describes a lubricating resin composition containing a base oil and a thickener, wherein a polyethylene oxide wax has been incorporated. Said reference indicates that it is used in the decelerators of electric power steering devices and the like. The decelerators of the electric power steering devices and the specific lubricated parts thereof are decelerator friction surfaces, comprising the synthetic resin worm wheel and metal worm shaft. However, said resin lubricating composition differs completely from the grease composition of the present invention.
The electric power steering device concerned in the present invention may conveniently be the device in the diagram appended to Japanese Laid-Open Specification 2003-335249. That is to say, said device may conveniently be a rack and pinion electric power steering device comprising a ball screw mechanism 37 and roller bearings 33 and 34 (wherein said numberings correlate to the device numberings stated in said reference), and wherein the power assist is effected in the axial direction by the ball screw connected to the rack shaft. Since this ball screw mechanism resembles the ball screw mechanism mounted in machine tools, the lubricating greases previously used on these parts were the lithium-based greases commonly used in these machine tools.
Further, the rack assist electric power steering device disclosed in Japanese Laid-Open Specification 2003-335249 has the electric motor arranged coaxially with the rack shaft. However, there are also devices wherein the electric motor is arranged not coaxially but parallel to the rack shaft (for example, as in Japanese Laid-Open Specification 2004-114972), and devices wherein the electric motor and the rack shaft are arranged so as to intersect the shaft centre (for example, as in Japanese Laid-Open Specification 2004-122858). In these devices, the electric motor and the ball screw mechanism (ball screw nut) are connected by a means of transmission such as a gear mechanism or a belt.
The ‘roller bearings’ in the aforesaid electric power steering devices are mainly single row or double row deep groove ball bearings (for example as in Japanese Laid-Open Specification 2004-144118).
However, with the ever-increasing demand for installation in cars, and the increased assist power, improved durability and low torque properties of electric power steering devices and the striking advances in other characteristics thereof, satisfactory durability could no longer be obtained with the lithium-based greases that have been hitherto widely used, stable steering properties from high to low temperatures could no longer satisfactorily be provided, and there were also problems with the low torque properties in lower temperature environments.
Accordingly, it is highly desirable to be able to develop urea-based lubricating grease compositions exhibiting stable low torque properties over a wide temperature range, whose effects can be seen to a marked extent at low temperatures, and with which long-term lubrication can be obtained with no lubricating film breakdown even at high temperatures, and a roller bearing and electric power steering device wherein the said lubricating grease composition is used as the lubricant.